In commercial chemical plants, solvents frequently must be evaporated out of a compound mixture or components must be reacted. If the products are sensitive to heat and, in particular, are of high viscosity, it is difficult to obtain a high output without thermal damage to the product.
Falling film evaporators are known, in which a liquid compound mixture flows in a thin layer along the walls of vertically arranged pipes under the influence of gravity. It is difficult to distribute a mixture over pipes connected in parallel since different distributions easily lead to faults in wetting and thus to local overheating of the product. Movement within the layer of liquid is slight, so only moderate exchange of materials and heat takes place. The falling film evaporator fails at higher viscosity since gravity is no longer adequate as conveying agent.
Thin layer evaporators are also known in which a viscous liquid is distributed in a thin layer along the internal walls of a generally cylindrical body by means of rotor blades in order to carry out the processes. Due to the moving parts, the thin layer evaporator is susceptible to breakdown and demands high maintenance costs. The operating costs are also high. The heat exchange is limited as it takes place essentially only over the external walling.
A spiral tube evaporator is also used for evaporation, stripping and reacting. In this apparatus, a thin annular flow is propelled in a continuously spiral pipe by gas which comprises solvent vapors and/or other foreign gas.
The propelling force for the annular flow is proportional to the viscosity of the gas, the wetted internal surface of the pipe and the speed of the gas, and is inversely proportional to the square of the diameter of the pipe. The method is not therefore particularly effective in the case of a small concentration of volatile components without the addition of other gas. The heat exchange takes place by means of the walling and the gas, the gas only being able to exchange energy once during the entire passage. Charactertistic of these coiled tubes are those described in U.S. Pat. No. 4,119,613; German Offenlegungsschriften 2,719,968; 2,719,972; 2,719,956; 2,719,967 and 2,719,969; and U.S. Pat. No 4,143,072 issued Mar. 6, 1979.
A method and an apparatus are required which allow evaporation, reaction and stripping, in particular of viscous materials, without moving components, in which a high heat exchange is ensured with at the same time, careful thermal treatment of the product and in which large product throughputs are economically processible with small amounts of gas.